Teledyne LeCroy Eye Doctor II - Manual (Page 41)

Operator's Manual
Finally the last step in the case 2 procedure is to Train the DFE processor. When we do this it clearly does
not succeed - the result, shown as EDrEqu and in the Eye diagram, does not change.
If we go to the DFE menu, and touch Edit/View DFE Setup, we see all the taps are 0.0 (the default), which
does nothing.
The training fails because too many bits are decoded incorrectly in the first pass and the beginning of
training always starts with all taps set to 0.
The problem is the received signal is so degraded that receiver equalization is having trouble dealing with
it. The usual way to address this problem is to split responsibility for equalization between the trans-
mitter and the receiver.
For this procedure, we add 6dB of de-emphasis to this waveform, which simulates TX equalization. We
also reduce the CTLE boost on the Clock Recovery path to 4dB, to compensate (we don't want to over-
equalize on the clock recovery path).
With DFE turned off, the eye diagram shows only the effect of 6dB of de-emphasis:
Case 3, with added 6dB de-emphasis (simulating TX equalization).
While much improved by 6dB of de-emphasis, the eye is just barely open. We can be sure the DFE can
Train on this. Now, we enable DFE and Train it again, producing the following result:
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